'If that person is working in a shop, perhaps you'll get the wrong change [but] if it's a driver then lives are at risk.”

London Underground's chief operating officer Steve Griffiths said in the last 12 months drivers have had an attendance rate of more than 94 per cent.

He said staff on sick leave were treated "with fairness and consistency" but that "failure to meet the standards required will result in disciplinary action or termination of service."

The disclosure comes as Transport for London as delayed the launch of 24-hour Tube services - originally set to begin this weekend - following a dispute with unions over changes to staff pay and working conditions.

Strike action due this week was called off and negotiations are still underway.